When conducting postal interviews, the surveys are sent to the respondents with letter post. One advantage of postal interviews is that you can attach the product or communication material that you want tested. Hereby, the respondents can relate to the specific material, while they answer the survey, which is not possible with a method like a phone interview.
Unlike a phone interview, the respondent can answer the postal interview when it fits their time schedule. Different from a web interview, postal interviews can include respondents who do not use the internet.
However, a disadvantage of the postal method, the data collection often takes longer than a method such as web interviews. Also, a bias on who would be inclined to answer the survey can be connected with postal surveys.